This invention pertains to thermally-stable additives for high-temperature fluids and more particularly to additives which impart anti-corrosion and anti-wear properties of said fluids.
A wide variety of lubricants and heat transfer fluids are known in the art. These materials can be paraffinic/naphthenic hydrocarbons, aromatic hydrocarbons containing one or more rings, silicones, fluorinated compounds, polyolefins, esters and the like. However, these fluids, in general, suffer from one or more difficiencies. Thus, for example, the paraffinic/naphthenic hydrocarbons are not thermally stable at or above 300.degree. C. Lubricants and fluids used in Rankine Cycle and heat transfer systems should be stable in the range of about 325.degree. to 375.degree. C.
Although silicones are thermally stable, they are relatively expensive and tend to decompose in contact with aqueous alkaline media.
Fluorinated compounds are also often quite stable but are also quite expensive.
Esters hydrolyze in aqueous media thus severely limiting their use.
Aromatic hydrocarbons having two or three rings have been found to be thermally stable, but are often exposed to conditions where the need for additives having anti-wear and anti-corrosion properties are needed. It is therefore an object of this invention to provide such additives, which themselves, are adequately thermally stable in the lubricants or heat transfer liquids for which they are used, that is, at temperatures in the range of about 325.degree. C. to about 375.degree. C.
A wide variety of additives are known in the art for different purposes such as reducing the wear of metals which fluids contact, to minimize corrosion, minimize oxidation, to improve viscosity-temperature characteristics, to reduce pour point and the like. Such compounds include for example, triethylenetetramine and various imidazolines which are employed as corrosion inhibitors. However these and similar compounds decompose at about 200.degree. C. Zinc dialkyl dithiophosphates and triarylphosphates, such as, tricresyl phosphate are extreme pressure anti-wear additives. However, these compounds are subject to hydrolysis in the presence of water and the zinc compounds decompose far below 300.degree. C. Fatty acids and fatty acid soaps, as for example, sodium laurate, are lubricity additives but these lack the stability required for this invention.
The term "high-temperature fluids" as used in this invention include heat transfer fluids, as well as lubricants for Rankine Cycle systems.
Other objects will become apparent to those skilled in the art upon reading of the specification.